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I/"* QOC1 BUREAU OF MINES 

|^ y^DI INFORMATION CIRCULAR/1990 



Frictional Ignition With Coal 
Mining Bits 



By Welby G. Courtney 




#+ >W X UiS> BUREAU OF MINES 



(80) 



1910-1990 



\ years g THE MINERALS SOURCE 

^U OF ^ 



Mission: As the Nation's principal conservation 
agency, the Department of the Interior has respon- 
sibility for most of our nationally-owned public 
lands and natural and cultural resources. This 
includes fostering wise use of our land and water 
resources, protecting our fish and wildlife, pre- 
serving the environmental and cultural values of 
our national parks and historical places, and pro- 
viding for the enjoyment of life through outdoor 
recreation. The Department assesses our energy 
and mineral resources and works to assure that 
their development is in the best interests of all 
our people. The Department also promotes the 
goals of the Take Pride in America campaign by 
encouraging stewardship and citizen responsibil- 
ity for the public lands and promoting citizen par- 
ticipation in their care. The Department also has 
a major responsibility for American Indian reser- 
vation communities and for people who live in 
Island Territories under U.S. Administration. 



Information Circular 9251 

Frictional Ignition With Coal 
Mining Bits 



By Welby G. Courtney 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 






> 



q^ 



\ 



TW 




Library of Congress Cataloging in Publication Data: 



Courtney, Welby G. 

Frictional ignition with coal mining bits / by Welby G. Courtney. 

p. cm. — (Information circular / Bureau of Mines; 9251) 

Includes bibliographical references. 

Supt. of Docs, no.: I 28.27:9251. 

1. Coal-cutting bits-Testing. 2. Friction. 3. Methane-Combustion. I. Title. 
II. Series: Information circular (United States. Bureau of Mines); 9251. 

TN295.U4 [TN813] 622 s--dc20 [622\334] 89-600350 CIP 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Experimental technique 5 

Results 6 

Rectangular bits 6 

Kennametal K-100 6 

Carmet TC3 10 

AMS THRU-FLUSH 12 

Hydra Tools HP74ISR 13 

Conical bits 14 

GTE 14 

Anti-ignition-modified cutter drum for Simmons Rand 265 continuous mining machine 16 

Kennametal K-178DC 17 

Discussion 18 

Conclusions 24 

References 25 

ILLUSTRATIONS 

1. Coal mine explosion 2 

2. Frequency of frictional ignitions in U.S. coal mines 2 

3. Shearer drum that caused frictional ignition of coal mine explosion in Nova Scotia 3 

4. Hot streak formed on surface of sandstone with worn bit 3 

5. Conventional- and mushroom-tipped conical bits 3 

6. Conventional- and dovetail-tipped rectangular bits 4 

7. Geometry of conical bit tip 4 

8. Effect of bit attack angle and initial tip angle on frictional ignition with conical bits 4 

9. Frictional ignition chamber 5 

10. Bit lacing of Joy ILS shearer drum 6 

11. Kennametal K-100 bit 6 

12. Effect of bit wear on frictional ignition with Kennametal K-100 bit 7 

13. Effect of bit velocity on frictional ignition with 0.45-cm-worn Kennametal K-100 bit 8 

14. Anti-ignition back-spray and bit geometry with Kennametal K-100 bit 8 

15. Hot streak cooled by back spray 9 

16. Artificial coal block containing sandstone slab 9 

17. Back sprays with fully laced Joy ILS shearer drum 9 

18. Carmet TC3 bit 11 

19. AMS THRU-FLUSH bit and test bit 12 

20. AMS THRU-FLUSH bit with fan back spray at 100 psig 12 

21. Hydra Tools HP74ISR bit 13 

22. Hydra Tools HP74ISR bit with cone back spray at 100 psig 13 

23. Hydra Tools HP74ISR bit with jet back spray at 100 psig 13 

24. Back spray and conical bit located at end of cutter drum 15 

25. New and 0.5- and 0.75-cm-worn GTE bits 15 

26. Anti-ignition-modified Simmons Rand 265 drum 17 

27. Modified Simmons Rand 265 drum in operation 17 

28. Kennametal K-178DC bit 17 

29. Kennametal K-178DC bit with solid-cone back spray at 150 psig 17 

30. Kennametal K-178DC bit with jet front spray at 150 psig 18 

31. Cutting processes with worn conical bit 20 

32. Spotty hot streak formed on surface of sandstone 21 

33. Theoretical temperatures inside conical bit and on wear-flat surface on conical bit 22 

34. Area of wear flat formed on steel-tipped conical bit versus number of cuts 22 

35. Geometry of linear wear distance with conical bit 23 

36. Linear wear distance versus cutting distance with conical bits 23 

37. Dust cloud formed by bit cutting sandstone 24 



TABLES 



Page 



1. Effect of bit wear on frictional ignition with Kennametal K-100 bit during dry cutting 

2. Effect of bit speed on frictional ignition with 0.45-cm-worn Kennametal K-100 bit during dry cutting 

3. Effect of Spraying Systems GG3004 back spray on frictional ignition with worn Kennametal K-100 bit 

4. Effect of Spraying Systems GG3004 back spray on frictional ignition with fully laced drum using 

0.54-cm-worn Kennametal K-100 bit 

5. Ignition results with Carmet TC3 bit and Senior Conflow back spray 

6. Ignition results with AMS THRU-FLUSH bit using fan-type back spray 

7. Ignition results with Hydra Tools HP74ISR bit with back spray 

8. Ignition results with GTE conical bit and Spraying Systems back spray 

9. Ignition results with Kennametal K-178DC bit 



7 
7 
9 

10 
11 
13 
14 
16 
18 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


°c 


degree Celsius 






mm 


millimeter 


cal/cm 2 • 


s calorie per square centimeter 


per 


second 


mm 2 


square millimeter 


cm 


centimeter 






/im 


micrometer 


cm 2 


square centimeter 






ms 


millisecond 


cm/min 


centimeter per minute 






pet 


percent 


cm/r 


centimeter per revolution 






psig 


pound per square inch gauge 


cm/s 


centimeter per second 






rpm 


revolution per minute 


deg 


degree 






rps 


revolution per second 


gpm 


gallon per minute 






s 


second 


gpm/cm 


gallon per minute per square 


centimeter 







FRICTIONAL IGNITION WITH COAL MINING BITS 

By Welby G. Courtney 1 



ABSTRACT 

This publication reviews recent U.S. Bureau of Mines studies of frictional ignition of a methane-air 
environment by coal mining bits cutting into sandstone and the effectiveness of remedial techniques to 
reduce the likelihood of frictional ignition. Frictional ignition with a m inin g bit always involves a worn 
bit having a wear flat on the tip of the bit. The worn bit forms hot spots on the surface of the sandstone 
because of frictional abrasion. The hot spots then can ignite the methane-air environment. A small 
wear flat forms a small hot spot, which does not give ignition, while a large wear flat forms a large hot 
spot, which gives ignition. 

The likelihood of frictional ignition can be somewhat reduced by using a mushroom-shaped tungsten- 
carbide bit tip on the mining bit and by increasing the bit clearance angle; it can be significantly reduced 
by using a water spray nozzle in back of each bit, which is carefully oriented to direct the water spray 
onto the sandstone surface directly behind the bit and thereby cool the hot spots formed by the worn 
bit. A bit replacement schedule must be used to avoid the formation of a dangerously worn bit. 



Supervisory research chemist, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



The frictional use of quartzitic and pyritic materials to 
make fire was known to Neanderthalers (I), 2 and frictional 
ignition has been called "about the second oldest profes- 
sion" (2). Frictional ignition of a violent coal mine ex- 
plosion such as shown in figure 1 was reported in 1675 (i). 

Field reports by the Mine Safety and Health Ad- 
ministration (MSHA) have indicated a general increase in 
the frequency of frictional ignitions in U.S. coal mines 
during the past years (fig. 2). While no miners have been 
killed in the United States because of frictional ignition 
since the early 1970's, injuries have occurred. The coal 
mine explosion in Nova Scotia in 1979 that led to 14 fa- 
talities was caused by a frictional ignition with the shearer 
drum shown in figure 3 (5). The increase in frictional 
ignitions in the United States thus is alarming. 

The MSHA field reports indicate that frictional ig- 
nitions in the United States almost always involve a metal 
bit cutting into sandstone. 3 Field samples of sandstone 
involved in frictional ignitions in Great Britain indicated 
that the sandstone must contain at least 20 pet quartz and 
usually contains 40 pet or more quartz (5). Early studies 
of frictional ignition reviewed in 1969 by Powell (6) 
indicated that frictional ignition with a metal bit cutting 
sandstone (1) involved a luminous hot streak that formed 
on the surface of the sandstone because of abrasion by the 

2 Italic numbers in parentheses refer to items in the list of references 
at the end of this report. 

Very occasionally frictional ignitions have appeared to involve a bit 
cutting into pyritic material. Also, a few incidents have involved 
sandstone-sandstone abrasion during the fall of a sandstone roof, a 
hand pick striking sandstone, and a drill bit striking sandstone or steel. 
One possible exception involved the cutting of "hard coal" (4). 



bit and did not involve hot, airborne, moving sparks, 4 
(2) always involved a worn bit that had formed a wear flat 
on the tip of the bit and was never obtained with a new, 
sharp bit, (3) could be obtained with almost any abrading 
material but was, e.g., much easier to obtain with steel 
than with a tungsten-carbide cermet (9), and (4) was 
apparently less likely to occur with a lower bit veloc- 
ity (5, 9-10). Figure 4 shows the hot streak formed on the 

4 Ignition by sparks can occur when the sparks adhere to a surface. 
However, references 7 and 8, e.g., conclude that ignition can be 
obtained with airborne, moving sparks. 




Figure 1.-Coal mine explosion. 



I00 



80 



Q 

u 60 



o 
a. 40 

UJ 

m 

3 



20 



KEY 
Continuous miner 

Shearer 

Roof bolter 

Other 



A 



L 



/ 



y_J£ 



n 



V\ V\ 



'/, 



m 



Z 



2 



n 



2 



V2 







M 



2 







V\ 



i i 



I * 



I97I I972 I973 I974 I975 I976 I977 I978 I979 I980 I98I I982 I983 I984 I985 I986 I987 I988 
Figure 2.-Frequency of frictional ignitions in U.S. coal mines. 





Figure 3.-Shearer drum that caused frictional ignition of coal 
mine explosion in Nova Scotia (3). 



Figure 4.-Hot streak formed on surface of sandstone with 
worn bit 



surface of a sandstone block by a downward-moving, worn 
rectangular bit having a wear flat. 

All mining bits presently use tungsten-carbide tips to 
reduce bit wear, but the carbide tips usually are small. 
The U.S. Bureau of Mines (11) designed large mushroom- 
shaped carbide tips for conical bits and dovetail-shaped 
carbide tips for rectangular (radial) bits to physically 
protect the steel shank and thereby reduce the likelihood 
of frictional ignition due to abrasion by the incendive steel 
shank. Figure 5 shows conventional- and mushroom- 
tipped conical bits. Figure 6 shows conventional plug- and 
peak-tipped rectangular bits and a mushroom-type 
dovetail-tipped bit. Laboratory tests indicated that the 
mushroom- and dovetail-tipped bits had longer wear lives 
than the conventional-tipped bits before the incendive steel 
shank became exposed (12). An in-mine test with conical 
bits supported these laboratory conclusions (13). 

Bureau tests have indicated, however, that frictional 
ignition can eventually be obtained with the wear flat 
formed on the tungsten-carbide tip of a nonrotating 
(frozen) conical bit without exposure of the steel shank. 
The likelihood of frictional ignition was significantly 
reduced with an increased initial bit clearance angle 9 C , 





Conventional tip Mushroom tip 

Figure 5. -Conventional- and mushroom-tipped conical bits. 



e.g., with conical bits, with an increased initial bit attack 
angle 9 A and/or a decreased internal bit tip angle T as 
shown in figure 7, where 6 A is the angle between the bit 
axis and the sandstone surface and 6 C = A - 6 T /2 (14). 
Figure 8 shows that the number of cuts to give ignition 
with a new, tungsten-carbide-tipped, frozen conical bit 
cutting into a sandstone block increased by a factor of 




Peak tip 




Dovetail tip 



Figure 6.-Conventlonal- and dovetail-tipped rectangular bits. 



Bit axis 



Bit motion 




• Sandstone- 



Figure 7,-Geometry of conical bit tip. (e A = attack angle; 
= initial clearance angle; 8j. = initial tip angle.) 




45 50 55 60 65 70 
BIT ATTACK ANGLE (0J,deg 



75 



Figure 8.-Effect of bit attack angle and initial tip angle on 
frictional Ignition with conical bits. 



about 3 if 6 A increased by 10° or T decreased by 10°. An 
in-mine test by Jim Walter Resources (JWR) (15) support- 
ed these laboratory conclusions. In a section prone to fric- 
tional ignitions because of a sandstone parting, no ignitions 
occurred in mining 50,000 tons of coal with mushroom bits 
having a Bureau-recommended bit attack angle of 57°, 
while two ignitions then occurred in mining 10,000 tons 
using conventional-tipped bits having the normal mine at- 
tack angle of 50°. 

While both of these features (mushroom tip and in- 
creased bit clearance angle) are valuable in reducing the 
early ignition hazard associated with conical bits, both fea- 
tures merely postpone the time when the bits have been 
worn to a dangerous condition likely to cause frictional 
ignition. A replacement schedule for worn bits is there- 
fore required. E.g., the bits in the JWR field test (15) 
were replaced after every shuttle car was loaded if the 
mining machine was cutting into a sandstone parting. 

Other laboratory studies (16-17) have indicated that a 
solid-cone water spray impacting onto the surface of the 
freshly cut sandstone directly in back of the cutter bit very 
significantly reduced the likelihood of frictional ignition, 
even with very worn bits, by promptly cooling the hot 
streak formed by the worn bit cutting the sandstone. The 
water requirement to reduce ignition was about 0.5 gpm 
for each bit and thus was somewhat high but not excessive. 



the use of water that impinges onto the front of the bit to 
reduce airborne respirable dust has become a standard 
mining practice in Great Britain. However, laboratory 
tests indicate that it had negligible effect in reducing 
frictional ignition (18). 

As part of its program to enhance the safety of workers 
in the mining industry, the Bureau has studied the problem 
of frictional ignition and especially the use of water sprays 
in back of the cutter bits to reduce the likelihood of 
frictional ignition. This report reviews several Bureau- 
funded studies conducted from 1979 to 1988. A detailed 
laboratory study with a rectangular bit and several types of 
water sprays was conducted by Bituminous Coal Research 
(now, Bituminous Coal Research National Laboratory 
(BCRNL)) from 1979 to 1981 under Bureau contract (18). 



In-house Bureau studies of a second back spray and 
rectangular bit system and a back spray and conical bit 
system were conducted in 1983. and a study of a front 
spray and conical bit system was conducted in 1987. Brief 
Bureau studies of several commercial back-spray systems 
with rectangular and conical bits were conducted in 1983 
and 1987. Results of these studies have not been 
previously published. A field test with back sprays and 
conical bits on a wet-head-modified cutter drum on a 
Simmons Rand 265 5 continuous mining machine was com- 
pleted in late 1988. These studies generally had a practical 
orientation. Recent Bureau work having a more funda- 
mental orientation is briefly presented in the "Discussion" 
section. 



EXPERIMENTAL TECHNIQUE 



Previous laboratory techniques to investigate frictional 
ignition have involved (1) a worn field bit making a single 
centimeter-deep spiral cut into new locations on a rotating 
block of sandstone (5), (2) a metal tool (9) or a worn 
rectangular field bit (10) making multiple shallow cuts at 
the same location on a stationary sandstone block, and (3) 
miscellaneous techniques such as a metal tool held against 
a grinding wheel, drill bit, falling weight, hand pick, and 
high-speed projectile (6). 

The miscellaneous techniques were used mainly to indi- 
cate qualitatively whether frictional ignition could occur. 
Techniques 1 and 2 were used mainly to indicate the 
effectiveness of a remedial technique in reducing the 
likelihood of frictional ignition. E.g., in investigating the 
effect of bit velocity, a worn bit making a deep spiral cut 
in a sandstone block (5) gave ignition in 3 s with a bit 
velocity of 150 cm/s and 0.3 s with a bit velocity of 
450 cm/s. The metal tool making multiple shallow cuts at 
the same location in a sandstone block (9) gave ignition 
with about 175 cuts with a tool velocity of 150 cm/s and 
75 cuts with a tool velocity of 450 cm/s. The worn bit 
making multiple shallow cuts at the same location (10) 
gave ignition with about 350 and 240 cuts with bit velocities 
of 100 and 280 cm/s, respectively. The investigators re- 
commended that a lower bit velocity be used to reduce 
the likelihood of frictional ignition. 

However, the relationship between these cutting 
techniques and field cutting is not clear. It was thought 
advisable to simulate field cutting insofar as reasonably 
possible. A new laboratory frictional ignition apparatus 
was developed by BCRNL (18). A single field bit was 
mounted onto a field cutter drum rotating downward in 
the vertical plane at a typical field drum rotation speed. 
A 51-cm-high block of sandstone containing 78 pet quartz 
(from Cleveland Quarries, Berea, OH) was positioned on 
a cart, with the bedding plane horizontal, and was moved 
horizontally across the rotating drum. A series of slanted 
downward cuts thus were made in new parts of the sand- 
stone block. The block was precut to excavate the circular 
arc in the block and was positioned so that a 0.6-cm-deep 



cut was made by the bit along the entire length of the arc. 
The block and drum were enclosed in a plywood chamber 
containing a 7-pct methane-air environment. The chamber 
included plastic blowout panels to relieve the chamber 
pressure when ignition occurred. Figure 9 shows the igni- 
tion chamber. 

The number of cuts made by the bit to ignite the 
methane-air environment was counted. The number of 
cuts during 1 pass of the block depended upon the cart 
speed and drum rotation speed and usually was about 20. 
If no ignition occurred during passage of the block, the 
block usually was redressed, repositioned, and again pass- 
ed across the rotating drum. The total number of cuts 
to obtain ignition was taken as a measure of the ease of 



s Reference to specific products does not imply endorsement by the 
U.S. Bureau of Mines. 




Figure 9.-Frlctional ignition chamber. 



ignition. The number of cuts to obtain ignition when a 
remedial technique was used, such as a lesser drum 
rotation speed or a water spray, then was measured. The 
increase in the number of cuts required to obtain ignition 
with the remedial technique was taken as a measure of the 
effectiveness of the remedial technique. 

The present cutting technique is similar to the tech- 
nique reported later in reference 19, in which multiple 
deep cuts with a worn field bit mounted on a field shearer 
drum rotating at a field speed were made in a sandstone 
block being moved horizontally below the drum. 



The initial laboratory study described below used a wet- 
head shearer drum fabricated for a Joy Technologies, Inc., 
ILS shearer. Subsequent laboratory studies used a seg- 
ment of the cutter drum from a used Joy 12 ripper-type 
continuous mining machine, which was modified here to be 
a wet head by using a small-diameter water seal attached 
to the end of the drum shaft. The field study used a wet- 
head cutter drum on a Simmons Rand 265 ripper machine 
that was designed and fabricated by Simmons Rand and 
used large-diameter water seals from Cannings Seals in 
Great Britain. 



RESULTS 



Six laboratory studies with water sprays were conducted, 
four with different rectangular bits and two with different 
conical bits. A third study with conical bits and back 
sprays mounted on the wet-head Simmons Rand 265 con- 
tinuous mining machine was conducted in the field. 

RECTANGULAR BITS 

Kennametal K-100 

In 1979, the Bureau initiated a contract with BCRNL 
(18) (1) to conduct a laboratory study of frictional ignition 
and especially the effectiveness of water sprays in reducing 
the likelihood of frictional ignition with rectangular cutting 
bits on a shearer drum and (2) to initiate and monitor a 
subsequent field study. A 76-cm-wide wet-head cutter 
drum for a Joy ILS shearer expected to be available for 
laboratory tests at the Bureau's Bruceton facility was 
designed by BCRNL and AMS Technology and fabricated 
by AMS. The drum used 43 bits and a tight lacing pat- 
tern (fig. 10) designed for difficult cutting conditions. 
Scheduling difficulties prevented use of the Joy ILS shearer 
at Bruceton, so the test chamber described earlier was 
used with the ILS drum. 

Kennametal, Inc., K-100 bits having a 2.5-cm-wide tip 
were used here. Figure 11 shows the side view of the 
K-100 bit. The bit had the conventional, peak-shaped, 
tungsten-carbide and cobalt cermet tip and a shank made 
of AISI Type 4140 steel. New bits and bits that had been 
shortened by 0.04 to 0.56 cm and ground flat to simulate 
a worn field bit were used. Bit wear during a test was 
negligible. The tip-to-tip diameter was 137 cm. A drum 
speed of 47 rpm was used in most of this study, giving a 
tip velocity of 337 cm/s. A cart speed of 41 cm/min was 
usually used, giving about 60 cuts during 1 pass of the 
51 cm-wide sandstone block, with one-third of the 2.5-cm- 
wide bit tip cutting into fresh sandstone and two-thirds 
of the tip abrading the previously cut portion of the 
sandstone. 

Initial work used a single bit. The hot streak formed on 
the surface of the sandstone by a worn K-100 bit cutting 



into the sandstone block was shown in figure 4. Airborne 
hot sparks were also formed, but motion picture and video 




672 mm 

762 mm 



90 mm 



Figure "lO.-Bit lacing of Joy ILS shearer drum. Circled x's 
denote mining bits. 




Figure 1 1 .-Kennametal K-100 bit 



photography indicated that ignition, with one exception, 6 
occurred at the bottom part of the hot streak. 

The number of cuts to give frictional ignition depended 
upon the amount of wear of the bit tip. The average num- 
ber of cuts to obtain ignition with worn bits is given in 
table 1 and shown in figure 12. Raw data are given in the 
footnotes in table 1. Results were rather scattered. Data 
considered to be probable "outliers" due to cutting into 
damp sandstone (see section on GTE bits) are enclosed 
in brackets in table 1 and were ignored in calculating the 
average number of strikes. Excluding outliers, the results 
still had considerable scatter; i.e., the coefficients of 
variation of the sets of data in the footnotes of table 1 and 
also other sets of data presented later in this report ranged 
from 25 to 80 pet. 

Table 1 .-Effect of bit wear on frictional ignition with 
Kennametal K-100 bit during dry cutting 

Bit shortening, Av number of cuts 

cm for ignition 

*>217 

0.08 2 >293 

0.16 3 45 

0.27 4 5.8 

0.32 V.O 

0.40 6 3.3 

0.45 7 5.5 

0.48 8 3.6 

0.54 9 2.2 

x No ignition with 150, 180, 230, 230, 250, 260. 

2 No ignition with 205, 255, 304, 306, 310, 312, 312, 312, 318. 
2, 3, 9, 14, 17, 28, 40, 50, 69, 79, 82, 91, 100, 
[143]; no ignition with [207, 328]. 

4 4, 4, 7, 8, [22, 120]. 

5 2, 3, 3, 3, 3, 4, 4, 5, 5, 6, 6, 6, 9, 9, 11, 12, 12, 12, 13, 16, 
[34, 38, 39, 50, 58, 60, 61]. 

6 2, 2, 4, 5, [14, 42, 57]. 

7 4, 4, 6, 8, [13]. 

8 2, 2, 2, 4, 4, 4, 5, 6, [14, 91]. 

9 1, 1, 1,3,3,4, [17,22]. 

No ignition was obtained with 217 dry cuts with a new 
bit or 293 cuts with a 0.08-cm-worn bit. Ignition was 
obtained with a slightly more worn bit, e.g., with an aver- 
age of 45 dry cuts with a 0.16-cm-worn bit and 5.8 dry cuts 
with a 0.27-cm-worn bit. The steel shank, which became 
exposed when the bit was worn 0.14 cm, presumably was 
the major contributor to ignition. The physical differ- 
ence between a "safe" 0.08-cm-worn bit and a "dangerous" 
0.27-cm-worn bit was only barely discernible in the 
laboratory, and a bit replacement schedule in the field 
probably could not be based on visual observation of the 
extent of bit wear or exposure of the steel shank. 

The likelihood of frictional ignition with a worn bit was 
not decreased with a lower drum rotation speed in the 

^n one test, ignition occurred below the block and presumably was 
due to the hot sparks adhering to the floor of the chamber. 





JUU 




• 1 1 1 


1 1 




z 




- 






- 


o 


II 








1- 
z 


200 


- 


Tungsten j 


Steel shank 


- 


o 






carbide | 


exposed 




DC 






1 






£ 


50 




1 




- 


C/5 








KEY 




h- 


40 






• No ignition 


_ 


O 








o Ignition 




Ll_ 












o 


30 








- 


rr 












UJ 












cu 


?0 








— 


:> 












-} 












?• 














10 






0"* 1 * 2 — ^_ 










1 1 1 


1 ^1 ° 






0.2 0.3 0.4 

BIT WEAR, cm 



0.5 



0.6 



Figure 12.-Effect of bit wear on frictional ignition with 
Kennametal K-100 bit 

range considered to be of practical interest. Results ob- 
tained with a 0.45-cm-worn bit during dry cutting are given 
in table 2 and plotted in figure 13. Ignition was obtained 
with about the same number of cuts when the drum was 
operated with a bit velocity of 108 or 337 cm/s (15 or 
47 rpm). The likelihood of frictional ignition did rapidly 
decrease (the number of cuts for ignition rapidly 
increased) with a still lower drum speed, but such a low 
drum speed may be impractical for underground mining. 

Table 2.-Effect of bit speed on frictional ignition 

with 0.45-cm-worn Kennametal K-100 

bit during dry cutting 

Drum speed, Bit speed, Number of cuts 
rpm cm/s for ignition 

47 337 ^.S 

38 272 12 

27 194 4, 6 

21 150 14 

17 122 8 

15 108 7 

13 93 >117 

9 65 >32 

^e table 1. 

The effect of water sprays from various water spray 
nozzles mounted in front or in back of a worn bit was 
investigated. With a water jet impinging onto the front of 
a 0.32-cm-worn bit, a 0.2-gpm jet was ineffective in 
preventing ignition; i.e., ignition occurred with nine cuts 
during wet operation versus seven dry cuts (table 1). A 
1.1-gpm front jet was more effective, in that ignition 
occurred with 69 wet cuts versus seven dry cuts. 

A back-mounted spray nozzle whose spray impacted the 
back of a 0.48-cm-worn bit was not very effective. With 



120 



h- 110- 



QC 40 
O 



O 20 

(T 
UJ 

2 10 



KEY 

• No ignition 
o Ignition 




See table 1 



1 



200 300 

BIT VELOCITY, cm/s 



400 



Figure 13.-Effect of bit velocity on frictional ignition with 
0.45-cm-worn Kennametal K-100 bit 



a water jet, ignition was obtained with 10 wet cuts, while 
ignition was obtained with an average of 3.6 dry cuts (ta- 
ble 1). A fan-type spray (Spraying Systems Co. W1502) 
was effective when the fan spray nozzle was operated at 
100 psig because the spray splashed off the back of the bit 
and impacted the sandstone surface within about 5 cm 
behind the leading edge of the tip of the bit. However, the 
fan spray was ineffective if the nozzle was operated at 
140 psig because the splash impact zone then was further 
behind the bit; i.e., impacting the hot streak with the water 
spray within about 5 cm behind the bit tip appeared to be 
required to avoid ignition. A solid-cone spray that im- 
pinged onto the back of the bit similarly gave only a 
modest anti-ignition benefit; i.e., ignition occurred with 
13 wet cuts compared with 3.6 dry cuts. 

A fan spray impinging the freshly cut sandstone surface 
directly behind the bit, with the fan oriented parallel to 
and directed onto the bit path, was somewhat effective. 
E.g., a 0.3-gpm fan spray did not permit ignition with a 
0.48-cm-worn bit with 120 wet cuts, while ignition was 
obtained with 3.6 dry cuts. However, ignition was obtained 
with 90 wet cuts with a 0.80-cm-worn bit. Also, the fan 
spray was ineffective when oriented perpendicular to the 
bit path. Since the fan spray appeared to be of only 
modest benefit with a very worn bit and especially since 
maintenance of a parallel orientation in the field was 
expected to be difficult because of nozzle misorientation, 
no additional work was done with fan sprays. 

A cone spray was selected for further work to avoid the 
nozzle orientation problem. A solid-cone spray was pre- 
ferred to a hollow-cone spray in order to give better 
cooling of the hot streak; i.e., the solid circle of drops at 
the impaction plane obtained with a solid-cone spray was 
expected to give better cooling than the annular ring of 
drops obtained with a hollow-cone spray. Spray nozzles 
giving a mean number drop size of about 200 /jm, con- 
sidered to be optimum for impaction coverage of a target 
surface (20), were chosen. Only Spraying Systems nozzles 




GG3004 
nozzle 



Figure 14.- Anti-ignition back-spray and bit geometry with 
Kennametal K-100 bit 



were considered in this study since the manufacturer 
provided information on mean drop size. The Spraying 
Systems nozzles considered included the GG3 nozzle, with 
an internal spray angle of 60° (0.9 gpm at 100 psig) and 
the GG3004 nozzle, with an internal spray angle of 30° 
(0.5 gpm at 100 psig). 

The GG3 spray at 70 psig did not give ignition with 
115 wet cuts with a very worn bit, while ignition occurred 
with an average of 3.6 dry cuts. Water consumption was 
0.7 gpm and thus somewhat high. The GG3004 nozzle was 
selected for detailed work since it involved a lesser water- 
flow rate and was considered to be more practical for field 
mining operations. Also, a spray with a 30° internal angle 
gives a greater water density (e.g., gallon per minute per 
square centimeter) at the impaction plane on the sand- 
stone surface than a 60° spray with a similar flow rate and 
thus should give better cooling of the hot streak. 

The likelihood of frictional ignition with a very worn bit 
was significantly reduced when the spray from the GG3004 
nozzle located 13 cm behind the bit tip impinged onto the 
freshly cut sandstone surface directly in back of the bit. 
Figure 14 shows the spray-bit geometry used here, with the 
spray nozzle oriented 50° to the surface of the drum. At 
the impaction plane, the spray had a diameter of about 
6.4 cm and an area of 33 cm 2 . Impingement of about 
10 pet of the spray water onto the back of the bit was 
purposely selected here in the event that a smaller bit may 
be used in the field. The spray nozzle was recessed in a 
steel housing to protect the nozzle from damage by broken 
coal. 

Laboratory results with a GG3004 nozzle, showing the 
effect of waterflow rate and pressure on the number of 



cuts to give ignition with worn bits, are given in table 3. 
Ignition did not occur with 100+ wet cuts using 0.4 to 
0.6 gpm (70 to 110 psig). Figure 15 shows the effect of the 
back spray on cooling the hot streak and is to be com- 
pared with figure 4. The hot streak extended about 2 cm 
behind the bit and thus existed for about 7 ms before be- 
ing cooled by the back spray. Ignition did occur in four 
out of six tests (table 3, footnote 4) when the nozzle pres- 
sure was 40 psig (0.3 gpm), but this low-pressure spray 
was poorly formed and gave a poor impingement pattern 
on the sandstone surface. 

Table 3.-Effect of Spraying Systems GG3004 back spray on 
frictional ignition with worn Kennametal K-1 00 bit 



Bit wear, 




Spray 






Av number of cuts 


cm 


Row rate, gpm 


Pressure, 


psig 


for ignition 


0.40 


Dry 

0.44 

.50 






Dry 
70 
90 




'3.3 
2 >114 
3 >130 


0.45 


Dry 
.34 
.47 
.56 






Dry 
40 
80 

110 




2 5.5 
4 >65 
5 >98 
6 >119 


0.54 


Dry 
.44 






Dry 
70 




'2.2 
7 >120 



1 See table 1. 

2 No ignition with 102, 109, 131. 

3 No ignition with 97, 114, 178. 

4 15, 15, 43, 76; no ignition with 119, 122. 

5 30; no ignition with 119, 121, 121. 

6 No ignition with 115, 117, 120, 120, 124. 

7 No ignition with 108, 119, 121, 121, 121, 121, 121, 123, 126. 



The above tests used a single bit. With a fully laced 
drum, broken coal from nearby bits may interfere with the 
spray pattern and reduce the anti-ignition effectiveness of 
the back spray. This aspect was investigated using the 
drum fully laced with 43 bits making a sump-type cut into 
a 152-cm-high, 152-cm-long, 76-cm-wide block of artificial 



coal containing a 51-cm-high, 76-cm-long, 76-cm-wide 
sandstone slab in the bottom part of the block (fig. 16). A 
GG3004 spray nozzle was located behind each of the 
43 bits as shown in figure 14. Figure 17 shows several of 
the bits with their back sprays. Total water consumption 
was 22 gpm when the 43 sprays were operated at 100 psig. 
Bits worn 0.54 cm were used to subject the anti-ignition 
back sprays to a severe test. The drum and block were 
again enclosed in a wooden box containing a 7-pct 
methane-air mixture. The drum was operated at 47 rpm. 
Depth of cut usually was 0.079 cm/r. The time to give ig- 
nition was taken as a measure of the ease of ignition. 



Artificial coal 



Sandstone 
slab 




CJ1 

ro 

o 

3 



Figure 16- Artificial coal block containing sandstone slab. 




********) 




Figure 15.-Hot streak cooled by back spray. 



Figure 17.-Back sprays with fully laced Joy ILS shearer drum. 



10 



No ignition occurred during 50 s of dry cutting into the 
artificial coal part of the block. Results obtained with the 
drum cutting into the sandstone part of the block are given 
in table 4. Raw data are again given in the footnotes, with 
tests considered to be outliers given in brackets. The 
17 "dry" tests in brackets in footnote 1 were conducted 
shortly after a wet test and thus involved cutting into damp 
sandstone. The bracketed test in footnote 3 involved bits 
that were severely worn on their sides because of abrasion 
of the bit on the side of the sandstone slab; this test was 
considered to be an outlier since the back spray would not 
impact and cool the hot streak formed on the side of the 
sandstone slab. 

Table 4.-Effect of Spraying Systems GG3004 back spray on 
frictional ignition with fully laced drum using 
0.54-cm-worn Kennametal K-100 bit 



Depth 


Spray 


Av time 
for igni- 


Nozzles 


of cut, 


Row rate, Pressure, 


plugged 


cm/r 


gpm psig 


tion, s 




0.079 . . 


Dry Dry 


> 







0.37 40 


, 2 53 







.41 60 


3 >75 


9 




.47 80 


4 >73 







.47 80 


5 >65 


4 




.53 100 


6 >63 







.53 100 


7 >69 


9 




.67 170 


8 >68 







.82 240 


9 >92 


9 


0.19 ... 


Dry Dry 


10 13 







.53 100 


9 >54 





0.58 .. . 


.53 100 


9 >74 






'3, 6, 8, 9, 10, 12, 22, 30, [6, 10, 10, 11, 12, 14, 14, 16, 19, 24, 32, 
34, 34, 38, 42, 42, 48]. 
2 38, 68. 

3 [22]; no ignition with 72, 78. 
4 No ignition with 72, 74. 
5 No ignition with 50, 80. 
6 No ignition with 48, 52, 60, 60, 66, 70, 70, 78. 
7 48; no ignition with 75, 83. 
8 No ignition with 61, 74. 
9 1 test. 
10 12, 14. 

Ignition occurred with an average of 13 s during dry 
cutting into the sandstone slab. Ignition was not observed 
in 54+ s during wet cutting if the water pressure was 
60 psig or higher. Ignition was observed in 53 s when the 
water pressure was reduced to 40 psig. 

No ignition occurred (>65 s) when four adjacent spray 
nozzles were plugged, indicating that a back spray provided 
considerable anti-ignition protection in cooling the hot 
streaks formed by nearby bits whose spray nozzles became 
clogged. When nine adjacent spray nozzles were plugged, 
no ignition occurred in five of six tests (averaging >75 s), 
again indicating the considerable protection provided by a 
back spray to nearby bits. 

The depth of cut did not appear to be significant, in 
that ignition was also obtained in 13 s of dry cutting when 
the depth of cut was 0.19 cm/r. 

Since 47 rpm corresponds to 0.8 rps, a 13-s test 
corresponds to 10 cuts by a bit. These multibit results may 
be compared with the earlier single-bit results, which gave 
ignition with 2.2 dry cuts, and suggest that frictional 



ignition is less likely with a multibit system. However, the 
anti-ignition protection provided by back sprays with both 
systems appears to be very significant. Briefly summarized, 
the BCRNL results were— 

1. In dry cutting, 

a. No frictional ignition occurred using new or 
slightly worn bits but ignition readily occurred with a few 
cuts using slightly more worn bits; 

b. Decreased bit velocity (in the range considered 
to be of practical interest) had negligible effect on reduc- 
ing the likelihood of frictional ignition using very worn bits. 

2. In wet cutting with very worn bits, 

a. Frictional ignition readily occurred when the wa- 
ter spray impinged onto the bit; 

b. The likelihood of ignition was significantly re- 
duced if the spray from a Spraying Systems GG3004 nozzle 
impinged onto the freshly cut sandstone surface directly 
behind the bit when the nozzle was operated at 80 to 
240 psig (0.47 to 0.82 gpm), but ignition occurred when the 
nozzle was operated at 40 psig; 

c. The broken coal did not significantly interfere 
with the anti-ignition effectiveness of the back spray, 

d. No ignition occurred when four adjacent nozzles 
were plugged, and the likelihood of ignition was signifi- 
cantly reduced when nine adjacent nozzles were plugged. 

BCRNL recommended that a Spraying System GG3004 
spray nozzle giving a solid-cone spray be located behind 
each cutter bit, be carefully oriented so that the spray 
water impinged onto the freshly cut mineral surface directly 
behind the bit, and be operated at 80 to 100 psig (0.5 gpm) 
at the nozzle. 

An early field test with an anti-ignition-modified 
Joy ILS drum using back sprays that extended 03 cm 
beyond their steel protective housings failed because of 
breakage of the spray nozzles. Field tests with the anti- 
ignition-modified Joy ILS drum using the recessed spray 
nozzle shown in figure 14 were cancelled because of the 
unavailability of a field shearer. 

Carmet TC3 

A mining company purchased anti-ignition shearer 
drums with Carmet (Allegheny Ludlum Industries Co.) 
TC3 rectangular bits and specified back-mounted Senior 
Conflow, Inc., 280INC spray nozzles instead of the Spray- 
ing Systems GG3004 nozzles because of easier nozzle re- 
placement. The 280INC nozzle gives a solid-cone spray 
having about the same spray geometry and waterflow rate 
(0.4 gpm at 100 psig) as the GG3004 nozzle and was ex- 
pected to provide similar anti-ignition protection. 

However, several frictional ignitions occurred in the 
field with the anti-ignition-modified drums. The mine op- 
erator had experienced nozzle clogging and had attempted 
to avoid clogging by (1) modifying half of the 280INC 
nozzles so that they delivered a hollow-cone spray instead 
of the solid-cone spray and (2) increasing the water 
pressure to 300 psig instead of using the 80- to 100-psig 



11 



range recommended by BCRNL. Furthermore, the opera- 
tor planned to use Senior Conflow 777NC nozzles 
operated at 500 psig in the future. While the general spray 
characteristics of the 777NC nozzle are similar to those of 
the 280INC and GG3004 nozzles and should provide simi- 
lar anti-ignition protection at 100 psig, the anti-ignition 
performance of these nozzles at 300 and 500 psig was 
unknown. 

Numerous in-mine tests have indicated that nozzle 
clogging due to dirty water entering the mining machine 
can he easily avoided with a Bureau-designed nonclogging 
system (21), which involves a Y-type strainer, a hydro- 
cyclone, and a polishing filter. Rust and scale particles 
that form in the water channels inside the machine can be 
eliminated with screens installed in each spray nozzle. 
Otherwise, regarding modification 1, the anti-ignition 
performance of a hollow-cone spray had not been 
investigated but was expected to be less than that provided 
by a solid-cone spray because of lesser cooling of the hot 
streak by the annular drop-impaction pattern instead of the 
solid-circle pattern. Regarding modification 2, a nozzle 
operated at high pressure tends to give a fine mist instead 
of the coarse spray obtained when the nozzle is operated 
at lower pressure. Misting is undesirable from an anti- 
ignition viewpoint since the small mist drops rapidly lose 
their momentum and would be less likely to impact the 
sandstone surface and cool the hot streak. 

Laboratory tests with the Carmet TC3 bit were 
conducted by the Bureau to investigate the anti-ignition 
performance of the 280INC and 777NC nozzles, especially 
when operated at high pressure. Figure 18 shows the 
1.3-cm-wide Carmet TC3 bit. TC3 bits were shortened by 
0.32 and 0.43 cm and flattened to simulate field bits that 
had become worn in order to expose the incendive steel 



Carbide tip- 




shank and subject the back sprays to a severe test. 
Previous work (with the GTE bit described later) had 
indicated that ignition with sandstone that had been 
dampened by a previous wet test was more difficult, i.e., 
required more cuts for ignition. Therefore, a sequence of 
only two ignition tests were made per day. The first test 
was conducted dry to confirm that the sandstone was 
suitably dry. The second test was conducted wet. Tests 
were then stopped, and a space heater was installed in the 
chamber to dry the sandstone block overnight. A new dry- 
wet sequence of two tests was conducted the next day. 

Results are given in table 5. Frictional ignition with 
the 0.32-cm-worn TC3 bit was obtained with an average of 
4.0 dry cuts but was not obtained with about 65 wet cuts 
using either the 280INC nozzle operated at 100, 300, and 
500 psig (0.4, 1.1, and 1.1 gpm) or the 777NC nozzle op- 
erated at 100 and 300 psig (0.4 and 0.8 gpm). However, 
with the 0.43-cm-worn bit, ignition was obtained with 
19 and 9 wet cuts when the 777NC nozzle was operated at 
100 and 300 psig and in 1 of 2 tests performed at 500 psig 
versus the average of 2.4 dry cuts for ignition. 

Table 5. -Ignition results with Carmet TC3 bit 
and Senior Conflow back spray 



Bit 


Nozzle 


Spray 




Av number 


wear, 


Flow rate, 


Pressure, 


of cuts for 


cm 




gpm 


psig 


Ignition 


0.32 . . 


Dry 


Dry 


Dry 


U.o 




280INC 


0.4 


100 


2 >66 




280INC 


1.1 


300 


3 >65 




280INC 


1.1 


500 


4 >63 




777NC 


.4 


100 


5 >68 




777NC 


.8 


300 


6 >69 


0.43 . . 


Dry 


Dry 


Dry 


7 2.4 




777NC 


.4 


100 


8 19 




777NC 


.8 


300 


9 9.0 




777NC 


1.3 


500 


10 >36 



Figure 18.-Carmet TC3 bit 



*2, 2, 2, 3, 3, 3, 4, 4, 5, 7, 8. 
No ignition with 65, 68. 
3 No ignition with 62, 64, 68. 
4 No ignition with 62, 64. 
s No ignition with 68, 68, 68. 
6 No ignition with 68, 70. 
n Z, 2, 2, 2, 2, 3, 3, 3. 
8 1 test 
9 5, 9, 12. 

10 

4; no ignition with 68. 

As with the Kennametal K-100 bit, the physical differ- 
ence between the "safe" 0.32-cm-worn bit and the "danger- 
ous" 0.43-cm-worn bit when using back sprays was barely 
discernible in the laboratory; e.g., the lengths of the ex- 
posed steel shanks were 1.7 and 2.2 cm, respectively. 

Since the areas of the exposed steel shanks of the worn 
Carmet TC3 and the Kennametal K-100 bits are com- 
parable and the worn K-100 bit was easily protected with 
the Spraying Systems GG3004 nozzle operated at 0.5 gpm 
and 100 psig, the Senior Conflow 280INC and 777NC 
sprays as used here appear to be somewhat less protective 
than the Spraying Systems GG3004 nozzle. 

Operation of either the 280INC or 777NC nozzle at 
300 psig and especially at 500 psig would appear to offer 



12 



no anti-ignition advantage and would definitely be dis- 
advantageous in terms of high waterflow rate and the usual 
expense and inconvenience of operating at high water 
pressure. 

It may be that the several field ignitions with the anti- 
ignition drums involved nozzle clogging. 

AMS THRU-FLUSH 

Sometimes there may be insufficient physical space on 
the cutter drum to locate a spray nozzle on the drum 
directly behind each bit as shown in figure 14. The back- 
mounted anti-ignition spray nozzles then must be located 
to one side of the bit, or some other physical change in the 
nozzle-bit geometry must be considered. 

One approach is to install the spray nozzle directly in 
the back part of the bit shank. Such a bit is commercially 
available from AMS Technology and is called a THRU- 
FLUSH bit. AMS offers either a slot-type spray nozzle, 
which gives a fan spray whose orientation is fixed and 
directed onto the bit path, or a solid-cone spray. AMS 
reported that field tests indicate that the fan spray gives a 
significant reduction in respirable dust compared with both 
the AMS cone spray and a conventional boom-mounted 
water spray system. 

The anti-ignition performance of the fan-spray version 
of the THRU-FLUSH bit was investigated in the labora- 
tory. In initial work, the THRU-FLUSH bit was cut down 
1.3 cm and flattened to expose the steel shank behind the 
carbide section in the front part of the bit tip but still 
protect the recessed spray nozzle, thereby simulating a 
worn field bit. However, reproducibility was poor because 
the trailing steel section of the flattened tip became worn 
in an irregular manner. The front carbide section of the 
tip was cut down another 0.6 cm so that only the flattened 
steel shank was abrading the sandstone surface. Figure 19 
shows the 2.5-cm-wide THRU-FLUSH bit on the left and 
the modified bit used in the present ignition tests on the 
right. Figure 20 shows the THRU-FLUSH bit with the fan 
back spray operated at 100 psig. The water density in the 
spray shown in figure 20 was less near the bit tip than 
farther behind the tip. This spray inhomogeneity may have 
been due to a faulty spray nozzle, but this aspect was not 
examined. 

A 38-cm-wide, 51-cm-high sandstone block was used in 
these tests. The block was moved at 23 cm/min across 
the drum being rotated at 39 rpm, giving about 65 cuts 
during 1 pass of the block. About one-fourth of the bit tip 
cut into a fresh part of the sandstone block and three- 
fourths of the tip abraded the previously cut part of the 
block. 

Results are given in table 6. The outlier in footnote 2 
occurred when the side of the bit abraded the side of the 
sandstone block. Frictional ignition was obtained with an 
average of 3.7 dry cuts and was not obtained with 63+ wet 



Carbide tip 



Water 
channel 




New bit 



Test bit 



Figure 19.-AMS THRU-FLUSH bit (left) and test bit (right). 




Figure 20 -AMS THRU-FLUSH bit with fan back spray at 100 
psig. 



13 



cuts. The AMS fan spray thus significantly reduced the 
likelihood of frictional ignition with the THRU-FLUSH 
bit, although the water consumption of 1 gpm was some- 
what high. 

Table 6.-lgnition results with AMS THRU-FLUSH 
bit using fan-type back spray 



The anti-ignition performances of the Hydra Tools 
spray systems were investigated. The Hydra Tools 
tungsten-carbide tip was replaced with a steel tip having 
the same geometry in order to investigate the effectiveness 
of the back spray under severe conditions. A new steel tip 







Spray 






Av 


number of cuts 


Row rate, 


gpm 




Pressure, 


psig 


for ignition 


Dry 






Dry 






l 3.7 


0.9 






50 






2 >64 


1.1 






100 






3 >67 


1.1 






300 






4 >64 


1.1 






500 






S >63 



H, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 5, 7, 8, 10. 

2 [64]; no ignition with 64, 64. 
3 No ignition with 65, 68, 68, 68. 
4 No ignition with 62, 64, 67. 
5 No ignition with 62, 64. 

Hydra Tools HP74ISR 

A rectangular bit that incorporates a water spray nozzle 
mounted on the cutter drum directly behind the bit, with 
the spray passing through a channel in the bit block and 
impinging onto the mineral surface in back of the bit, is 
commercially available from Hydra Tools International, 
Inc. (HP74ISR). Spray nozzles giving solid-cone and jet 
sprays are offered. Figure 21 shows the 2.5-cm-wide bit. 
Figure 22 shows the cone version of the Hydra Tools back 
spray, and figure 23 shows the jet version, with both spray 
nozzles operated at 150 psig. 




Figure 22.-Hydra Tools HP74ISR bit with cone back spray at 
100 psig. 



Carbide tip- 





Figure 21. -Hydra Tools HP74ISR bit 



Figure 23.-Hydra Tools HP74ISR bit with jet back spray at 100 
psig. 



14 



was used in each test. The 51-cm-wide sandstone block 
was moved at 84 or 127 cm/min across the drum being 
rotated at 42 rpm, giving 26 or 17 cuts during 1 pass of the 
block, with 80 and 100 pet of the width of the cuts, respec- 
tively, made in fresh sandstone. The total number of cuts 
to obtain ignition with a new steel tip was counted. 

Results (table 7) did not depend upon cart speed. The 
three bracketed tests in footnote 1 involved cutting into 
damp sandstone. Ignition was obtained with an average of 
5.7 dry cuts. No ignition was obtained with the cone or jet 
spray with an average of 132 or more cuts. The cone 
spray perhaps could be considered less protective than the 
jet spray, in view of the two ignitions in three tests at 100 
psig, but additional replicate tests are required. The jet 
spray definitely involves considerably greater water 
consumption. 

Table 7.-lgnition results with Hydra Tools HP74ISR 
bit with back spray 



Nozzle 




Spray 






Av number of cuts 




Row rate, 


gpm Pressure, 


psig 


for ignition 


Dry . . . 


Dry 




Dry 




'5.7 


Cone . . 


1.3 




100 




2 >132 


Do ... 


1.6 




150 




3 >203 


Jet 


3 




100 




4 >155 


Do ... 


5 




150 




4 >182 



l 1, 2, 2, 2, 2, 2, 2, 3, 3, 5, 5, 5, 5, 6, 6, 6, 7, 8, 9, 11, 12, 13, 14; 
no ignition with [16, 17, 18]. 
2 61, 136; no ignition with 199. 



..,.._ ignition with 199 
3 No ignition with 102, 303, 
4 1 test 



The present results indicate that the Hydra Tools back- 
spray system is effective in reducing the likelihood of 
frictional ignition with a worn version of their bit. Of 
course, ignition with only a few dry cuts with the present 
steel-tipped bit does not indicate the ignition performance 
of the commercial Hydra Tools bit with its tungsten- 
carbide tip. 

CONICAL BITS 

About 75 pet of the frictional ignition incidents in U.S. 
coal mines (fig. 2) involved ripper-type continuous mining 
machines. Application of the back-spray concept to a 
ripper machine involves (1) the design and fabrication of 
a wet-head ripper drum using durable large-diameter 
water seals and (2) careful installation of properly 
engineered water spray nozzles behind each cutter bit. 

In the early 1970's several Bureau-funded field stud- 
ies (22) investigated the effect of drum-mounted water 
spray nozzles on respirable dust with wet-head ripper 
machines, using state-of-the-art water seals. Results 
indicated that water sprays mounted on the ripper drum in 
the general vicinity of the cutter bits reduced respirable 
dust compared with conventional boom-mounted water 
sprays and did not interfere with visibility or splash onto 



the machine operator. However, seal life was severely 
limited. A durable, large-diameter water seal was recently 
developed by Cannings Seals in Great Britain and has 
been successfully used in mining equipment in Europe and 
elsewhere for several years. A wet-head cutter drum for 
a Simmons Rand 265 continuous mining machine using 
Cannings water seals was designed and fabricated by 
Simmons Rand under Bureau contract J0395040. 

However, a ripper drum usually uses conical bits with 
a small bit attack angle while a shearer drum usually uses 
rectangular bits with a large attack angle. In addition, the 
bit and bit block of a conical bit may physically interfere 
with the back spray impacting the mineral surface immedi- 
ately behind the bit to a greater extent than occurs with a 
rectangular bit system such as described previously. Also, 
there may be insufficient physical space on the ripper 
drum to locate an anti-ignition spray nozzle directly behind 
each bit, and the back-mounted spray nozzle for a ripper 
drum may have to be located to one side of the bit and 
further away from the mineral surface. The design of an 
anti-ignition back-spray system for a conical bit on a ripper 
drum thus may be more difficult than for a rectangular 
(radial) bit on a shearer drum because of spray-bit 
geometry. 

A laboratory study of the anti-ignition performance of 
several types of back-spray nozzles with a typical conical 
bit using the wet-head segment of a Joy 12CM drum is 
described in the following section. A recent field study of 
a specific back-located spray nozzle with several types of 
conical bits using the fully laced Simmons Rand 265 wet- 
head drum is described later. 

GTE 

A commercial "double-carbide" GTE Corp. conical bit 
being used in a coal mine in which a future field test was 
expected to be performed was tested with the wet-head 
segment of the Joy 12CM drum. The commercial bit 
block gave a bit attack angle of 45°. The bit and bit block 
involved considerable interference with the back spray im- 
pacting the mineral surface immediately behind the bit. 
This interference was especially severe for a bit mounted 
on the end of the drum. 

The effect of spray parameters for a single conical bit 
mounted in the middle or at the end of the drum was in- 
vestigated. Water spray nozzles were located on the cutter 
drum in back of and, because of space limitations with the 
Joy 12CM as laced, to one side of the bit. The spray 
nozzles were about 10 cm from the tips of the bits. It was 
expected that a field drum would channel water through a 
scroll mounted on the outer surface of the drum. The 
spray nozzles were mounted 2 cm above the surface of the 
drum to simulate a field drum with scroll. A universal 
joint was used to facilitate nozzle orientation. Figure 24 
shows the end bit with its spray nozzle. 



15 





Figure 24. -Back spray and conical bit located at end of cutter 
drum. 




Figure 25.-New and 0.5- and 0.75-cm-worn GTE bits, left to 
right 



The base of the bit was cut with a slot that fit over a 
tongue in the bit block to prevent the bit from rotating 
during cutting. The tip of the bit was shortened by 0.5 or 
0.75 cm and then cut to have a flat surface to simulate a 
field bit that had become frozen and had worn a wear flat. 
The flattened bit purposely exposed part of the steel shank 
in order to submit the anti-ignition back spray to a severe 
test. The wear flat was about 2.5 cm wide and 3 cm long. 
Figure 25 shows a new bit on the left and 0.5- and 
0.75-cm-worn bits for the end location in the middle and 
on the right. A newly fabricated flattened bit was "worn 
in" with 100 cuts to match the flat on the bit to the exca- 
vated arc in the 51- by 51- by 51-cm block of sandstone. 
With the typical cart speed of 28 or 71 cm/min and 
the drum speed of 39 rpm used in the field, about 70 or 



29 cuts were respectively made in the block during 1 pass. 
With the slower cart speed, about 33 pet of the bit cut 
into sandstone and 67 pet abraded the previously cut sur- 
face of the sandstone. With the faster cart speed, about 60 
pet of the bit cut into sandstone and 40 pet abraded the 
previously cut surface. 

The sandstone used in these studies readily absorbed 
water, and frictional ignition with a damp sandstone 
surface that had been moistened by weather or a previous 
wet test was more difficult (required more cuts) than with 
a dry sandstone surface. In four instances involving a 
sequence of dry-wet-dry tests with a 0.75-cm-worn middle 
bit, the second "dry" test involved cutting into the 
dampened sandstone and required an average of 22.5 cuts 
for ignition instead of the average of 4.7 cuts in the first 
dry test using sandstone that had been dried overnight. 
Later tests therefore involved only one wet test per day, 
with the sandstone block then dried overnight with a space 
heater. 

The present wet results were very sensitive to the 
precise orientation of the back spray nozzle, in that 
changing the aim of the nozzle relative to the bit by only 
a few degrees significantly decreased the anti-ignition 
effectiveness of the back spray. This decrease appeared to 
be due to water impacting the bit and bit block; i.e., such 
water again seemingly was wasted in terms of anti-ignition 
performance. The spray nozzles were oriented initially by 
visually observing the spray impaction pattern on a plastic 
sheet held to simulate the impaction plane on the 
sandstone and later by using a template. 

Results are given in table 8 and reported in refer- 
ence 23. Bracketed tests in the footnotes were considered 
to be outliers due to cutting into damp stone or to a poor 
nozzle orientation. Ignition with worn bits again was 
readily obtained with only a few dry cuts. There was no 
drastic difference at the middle and end locations; e.g., 
with a 0.75-cm-worn bit, ignition was obtained with an 
average of 4.7 dry cuts with the middle bit and 5.0 dry cuts 
with the end bit. Results did not depend upon cart speed. 

As before, the likelihood of frictional ignition during dry 
cutting was not appreciably reduced when the drum speed 
was decreased; i.e., with the 0.75-cm-worn middle bit, 
ignition was obtained with 5 and 7 dry cuts in 2 tests when 
the drum was operated at 20 rpm compared with the 
average of 4.7 cuts when the drum was operated at 
39 rpm. 

Wet cutting with a carefully oriented back spray again 
significantly reduced the likelihood of frictional ignition. 
With 0.5-cm-worn middle and end bits, ignition was ob- 
tained with an average of 6.9 and 2.8 dry cuts and was not 
obtained with 60+ and 40+ wet cuts. Similarly, with 
0.75-cm-worn middle bits, ignition was obtained with 
4.7 dry cuts and was not obtained with 39+ wet cuts. 

The planned field test was canceled by the coal mine 
operator. 



16 



Table 8.-lgnition results with GTE conical bit and Spraying Systems back spray 







Spray 




Bit wear, 
cm 


Av number of cuts 


Nozzle 


Flow rate, 
gpm 


Pressure, 
psig 


Internal 
angle, deg 


for ignition 




Middle bit End bit 



Dry 

GG3 . . . . 
3002.5 . . 
3004 . .. 

TTD3-56 
TTD2-56 



Dry 
Dry 
Dry 
0.7 
.8 
.9 
.3 
.3 
.4 
.4 
.5 
.5 
.5 
.5 
.6 
.4 
.5 



Dry 

Dry 

Dry 

60 

80 

100 

60 

80 

110 

60 

80 

80 

110 

100 

150 

100 

150 



Dry 
Dry 
Dry 
60 
60 
60 
35 
35 
35 
30 
30 
30 
30 
25 
25 
20 
20 



New 
0.50 
.75 
.75 
.75 
.75 
.75 
.75 
.75 
.50 
.50 
.75 
.75 
.50 
.50 
.50 
.50 



z 6.9 
4 4.7 



'>69 

8 41.7 
>>47 
l >72 
2 >60 
! >39 
( >54 



5 > 67.2 

5 > 59.3 



'>47 
3 2.8 
5 5.0 
6 29 

s >42 

5 >24 



°>59 
} >20.5 



'>24 



16 



19 



14 
'34 



39.5 
l >69.5 



*24; no ignition with 70. 



2 3, 4, 5, 5, 6, 7, 10, 11, 11, [17, 23, 23, 27, 44, 59, 72]; no ignition with [50, 72, 76]. 

3 1, 1,2,7. 

4 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 9, [14, 14, 16, 16, 26, 32, 35]; no ignition with [36]. 

5 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 4, 4, 4, 5, 5, 6, 6, 7, 7, 7, 8, 8, 9, 9, 9, 10, 10, 14, [14, 14, 26, 28]; no ignition with [61]. 

6 1 test. 



7 66; no ignition with 69, 72 

8 28, 35, 62. 

9 [7], 29; no ignition with 65. 

10 15; no ignition with 26. 

u No ignition with 70, 72, 72, 73, 73. 

12 27; no ignition with 74, 80. 

13 23, 26, 26; no ignition with 81. 

14 14, no ignition with 24, 33, 58, 74, 75, 75. 

15 [3, 4, 6]; no ignition with 24. 

16 [3], 14. 

[6], 34. 

No ignition with 62, 67, 67, 67, 73. 

[5], 24, 27, 47, 60. 
^No ignition with 33, 67, 67, 70. 
21 No ignition with 69, 70. 

NOTE.-Dashes indicate no tests were performed under these conditions. 



17 



19 



Anti-Ignition-Modified Cutter Drum for 

Simmons Rand 265 Continuous 

Mining Machine 

The bit lacing for the wet-head Simmons Rand 265 rip- 
per machine was designed by AMS for moderate cutting 
conditions occurring in a specific mine that had a history 
of frictional ignitions and was expected to provide a field 
test site. The drum was laced with a total of 76 AMS 
conical bits, 72 bits in the center part of the drum and 2 
bits in the middle part of each end of the drum. The bit 
blocks gave a bit attack angle of 52°. Sufficient space was 
available to install water spray nozzles about 18 cm behind 
the tip of each of the 72 center bits. Figure 26 shows 
several of the center bits and their spray nozzles. No spray 
nozzles were used behind the four end bits because of 
engineering difficulties in channeling water to the end 
locations. While a channeling technique could un- 
doubtedly be devised, the present effort was limited by 
financial restrictions, and conveyance of the water to the 
end locations was postponed to a future effort if seal 
durability in the field could be demonstrated. 



The AMS solid-cone spray nozzle (06-900870-000) had 
a 20° internal angle and delivered a well-formed spray at 
0.3 gpm and 50 psig. Each nozzle was carefully oriented 
using a template so that about 75 pet of the spray water 
impacted the mineral surface directly behind the bit and 
25 pet of the water impacted the bit and bit block. This 
spray orientation led to wastage of the water impacted on 
the bit and block but was selected in view of the sensitivity 
of the anti-ignition performance to the spray-bit orienta- 
tion and the possibility that other bits might be used in the 
field. No laboratory ignition tests were done with this 
spray-bit system. 

The wet-head-modified Simmons Rand 265 machine 
was installed in the ignition-prone mine in mid- 1987 in a 
section having a 30- cm-high sandstone parting. Figure 27 
shows the modified drum in operation in the mine. Re- 
sults indicated no water leakage with the Cannings water 
seals during the mining of 150,000 tons, despite several 
early shifts of dry operation with no water being passed 
into the seals. Miscellaneous conical bits were used. No 
frictional ignitions have occurred, airborne respirable dust 
was dramatically reduced according to visual observation, 



17 




Spray 
nozzle 





i^ii'iiiiiiifpN. 

Figure 26.-Anti-ignition-modified Simmons Rand 265 drum. 



Scale, cm 
Figure 28.-Kennametal K-178DC bit 



X 



Figure 27.-Modified Simmons Rand 265 drum in operation 



bit life increased 50 pet, and the wet-head drum was well 
received by face personnel because of increased visibil- 
ity (24). The test was completed in mid- 1988 when the 
mining machine was removed for routine maintenance 
overhaul. Inspection of the seals indicated that they were 
still in good condition. 

Kenna metal K-178DC 

A conical bit that incorporates water spray nozzles 
directly in the bit block behind and/or in front of the bit 
is commercially available from Kennametal (K-178DC) 
(fig. 28). The bit attack angle is 55°. Kennametal offers 
either solid- or hollow-cone or jet spray nozzles. Figure 29 
shows the bit with the Kennametal solid-cone back spray 
(Senior Conflow 2801) operated at 150 psig. Figure 30 
shows the Kennametal jet front spray, also operated at 
150 psig. 




Figure 29.-Kennametai K-178DC bit with solid-cone back 
spray at 150 psig. 



The anti-ignition performance of the Kennametal back- 
and front-located spray systems was investigated. The 
anti-ignition benefit of a front-located high-pressure jet 
using a Bureau-designed 0.06-cm-diam nozzle orifice also 
was tested during other work investigating the enhance- 
ment of cutting (25). The Kennametal tungsten-carbide 
tip was replaced with a steel tip having the same geometry 
in order to subject the Kennametal sprays to severe test 
conditions. A new steel-tipped bit was used in each test. 



18 




Figure 30.-Kennametal K- 178 DC bit with jet front spray at 
150 psig. 



Results are given in table 9. Ignition was obtained 
with an average of 4.2 dry cuts and was not obtained with 
97+ wet cuts with the hollow- or solid-cone spray mounted 
in back of the bit. With the front-mounted Kennametal 
and Bureau jet sprays, ignition was obtained in about half 
of the tests at each of the cited pressures. 

The Kennametal back spray thus significantly reduced 
the likelihood of frictional ignition. Again, the present re- 
sults giving ignition with only a few dry cuts with the steel- 
tipped bit do not indicate the ignition performance of the 
commercial Kennametal bit with its tungsten-carbide tip. 

Table 9.-lgnition results with Kennametal K-178DC bit 





Nozzle 


Spray 


Av number 


Location 


Type 


Row rate, 


Pressure, 


of cuts for 






gpm 


psig 


ignition 


Dry . . . 


Dry 


Dry 


Dry 


U.2 


Back . . 


Hollow cone 


1 


150 


2 >97 


Do . . . 


Solid cone. . 


1 


100 


3 >138 


Front . . 


Jet 


.9 


150 


4 >18 


Do . . . 


Bureau jet. . 


.75 


2,000 


s >65 


Do . . . 


. . do 


.90 


3,000 


6 26 


Do . . . 


. . do 


1.15 


5,000 


7 >65 



! 1, 1,3,3,3,4,6,8,9. 

2 No ignition with 28, 107, 117, 137. 

3 1 test. 

4 9; no ignition with 28. 

5 10; no ignition with 121. 

6 1 7, 21,39. 

7 10, 13, 24, 35, 37; no ignition with 103, 122, 179. 



DISCUSSION 



Frictional ignition with the cutter bits on a mining 
machine can occur when the mining machine is cutting 
into sandstone (or pyrite). Conversely, if the mining 
machine is cutting into sandstone, the mine operator 
probably should expect frictional ignitions. 

Remedial techniques to reduce the likelihood of 
frictional ignition include 

1. Mushroom-shaped tungsten-carbide bit tips; 

2. Increased bit clearance angle, e.g., with conical bits, 
increased bit attack angle and/or decreased internal tip 
angle; and 

3. A carefully oriented water spray nozzle in back of 
each cutter bit. 

With a shearer drum, the preferred remedial technique 
is to use drum-mounted back sprays. Fabrication of an 
anti-ignition shearer drum involves moving the variously 
located antidust water spray nozzles on the commercially 
available wet-head shearer drum so that they are located 
directly behind each of the bits and are carefully oriented 
so their sprays impact the mineral surface directly behind 
the bits. Several anti-ignition back spray and bit systems 
are commercially available (e.g., AMS, Hydra Tools, 
Kennametal) and have been used in the field. An antidust 
benefit has been reported with the anti-ignition back spray. 



A wet-head cutter drum for the Simmons Rand 265 
continuous mining machine that incorporates anti-ignition 
drum-mounted back-located spray nozzles was developed 
by Simmons Rand and AMS. However, wet-head cutter 
drums for other continuous mining machines are not 
commercially available at present. In the interim, the 
operator can use the mushroom tip and increased 
clearance angle features for conical bits presently being 
used by JWR (15). Mushroom-shaped tips for conical bits 
are commercially available. 

The mine operator is cautioned that no anti-ignition 
remedial technique will be a panacea and that a systematic 
schedule must be used to replace bits before they have 
worn to a dangerous condition. The JWR schedule of 
replacing bits after every shuttle car is loaded when 
sandstone is being cut is inconvenient and expensive but 
is probably as important in avoiding frictional ignition 
as the use of mushroom tips and increased bit clearance 
angle (15). 

Ignition seemingly can be obtained with almost any tip 
material but appears less likely to occur (requires more 
cuts) with a harder, more durable material. With conical 
bits, ignition was typically obtained here with 5 cuts using 
a medium-hard steel tip and 50 cuts using a hard tungsten- 
carbide tip. Similar results were obtained with the more 
massive rectangular bits. Ignition with the Carmet TC3 bit 



19 



required about 10 cuts with a steel tip but was not 
obtained here with 200 cuts with a tungsten-carbide tip. A 
small, 1-mm 2 wear flat did form on the carbide tip, and 
ignition presumably would eventually have been obtained 
with the larger wear flat formed with additional cuts. The 
formation of a similar small wear flat on a synthetic 
diamond tip was observed here and also reported by 
Roepke (26). The wear rates of diamond- tipped and 
tungsten-carbide-tipped rectangular bits were not mea- 
sured here because of the low wear rates of the massive 
tips. However, field tests (27) indicated that diamond- 
tipped rectangular bits were significantly more durable 
than comparable tungsten-carbide-tipped bits and thus may 
be less likely to cause frictional ignition. 

The present observation that a lower bit velocity did not 
appreciably decrease the likelihood of frictional ignition 
with a worn bit until a very low velocity was used does not 
agree with previous studies (5, 9-10). This disparity 
perhaps is due to differences in bits or test procedures. 
The present procedure involved a moving bit that inter- 
mittently made deep cuts into new locations of a sandstone 
block, while the study reported in reference 5 used a 
stationary bit making a continuous deep cut into a new 
location on the moving sandstone block and the studies in 
references 9 and 10 used moving bits making shallow cuts 
into the same location on a stationary sandstone block. 
However, the present results indicate that a lower bit 
velocity probably is not a reasonable alternative to avoiding 
frictional ignition with worn bits in a practical mining 
operation. 

Improved ventilation is often proposed as a remedial 
technique to reduce the likelihood of frictional ignition. 
While improved ventilation reduces the severity of the 
methane explosion that results from a frictional ignition, it 
probably increases the likelihood of a frictional ignition. 
Methane is discharged from the face into the entry and 
mixes with the ventilation airstream in the entry. A 
boundary layer automatically forms at the face, wherein 
the local methane concentration is high at the face and 
decreases away from the face. A combustible methane- 
air zone therefore must be formed near the face. If the 
ventilation is increased, the total amount of methane in the 
boundary layer is decreased but the combustible methane- 
air zone is moved closer to the ignition source, the hot 
streak at the face. The net result is that the likelihood of 
a frictional ignition should increase as the ventilation 
increases, but the severity of the resulting methane ex- 
plosion should decrease; i.e., small pops occur instead of 
a big bang. 

The anti-ignition effectiveness of a back spray depends 
upon the severity of the hot streak, the spray density at 
the impaction plane on the sandstone, the cooling effec- 
tiveness of the spray drops, and the spray-bit geometry. A 
minimum water density at the spray impaction plane of 
about 0.015 gpm/cm 2 appears to be required to cool the 
hot streak formed by the rectangular Kennametal K-100 
bit during severe test conditions. With the spray and 
conical bit system used with the GTE bits, a minimum 
water density of about 0.007 gpm/cm 2 was required for the 



middle bit but 0.015 gpm/cm 2 was required for the end 
bit. With the AMS spray used on the Simmons Rand 
265 drum, the water density also was 0.007 gpm/cm 2 . Such 
water densities can be readily achieved with commercial 
water spray nozzles using modest flow rates and pressures, 
e.g., with the GG3004 nozzle operated at 0.5 gpm and 
100 psig or with the AMS nozzle operated at 0.3 gpm and 
50 psig. The use of a high waterflow rate of 1 or 2 gpm is 
commercially popular but does not appear to be required 
from an anti-ignition viewpoint. Powell and Billinge (17) 
recently reported that 90-^m drops were twice as effective 
as 200-/im drops in cooling the hot streak, and a future 
study of improved water sprays will include spray nozzles 
selected to give 90-/im drops. 

However, continued bit wear may give a hot streak that 
is not adequately cooled by this water density; therefore, 
a bit replacement schedule still should be used with a 
back-spray technique, but a less frequent bit replacement 
would probably be required. Also, although all back spray 
systems tested here successfully reduced the likelihood of 
frictional ignition by all bits tested here by a very signi- 
ficant degree, other bit geometries might prevent the hot 
streaks from being promptly cooled by back sprays and 
thereby allow frictional ignitions. These aspects have not 
yet been examined. 

The present observation that frictional ignition is less 
likely when the sandstone contains water is not surprising 
and was reported by Titman (28). However, the natural 
water in the sandstone does not appear significant; i.e., the 
ignition frequency in the field in the wet summer months 
was comparable to the frequency in the dry winter months. 
Infusion of the coal seam with water in order to reduce 
respirable dust should have an anti-ignition benefit, but the 
degree of benefit remains to be determined. 

From a more fundamental viewpoint, the ignition of a 
methane-air mixture by a hot surface depends upon the 
temperature and area of the hot surface and the exposure 
time. With an electrically heated surface and a 1-s 
exposure time, Rae (29) reported that a surface tempera- 
ture of about 1,400° C was required for ignition with a 
10-mm 2 surface while 1,250° and 1,200° C were required 
with 50- and 100-mm 2 surfaces. Similar results were re- 
ported by other investigators (30-33), but the required sur- 
face temperature increased as exposure time decreased; 
e.g., with a 10-ms exposure time, 1,900° and 1,850° C were 
required with 50- and 100-mm 2 surfaces. 

When a metal bit cuts into a material such as sand- 
stone, frictional abrasion leads to the formation of a wear 
flat on the tip of the bit and the generation of heat by the 
wear flat. Following Osburn (34), cutting involves a 
sequence of chipping-crushing-abrading processes such as 
shown in figure 31, where the tip of the wear flat cuts off 
a chip of the sandstone, and the wear flat, which has a 
wear angle, then crushes the remaining layer of the sand- 
stone and also abrades the newly formed sandstone parti- 
cles against the sandstone substrate. It is generally 
thought that the surfaces of the wear flat and the sand- 
stone become heated by abrasion between asperities until 
the melting temperature of the lower melting material is 



20 



-Bit axis 




i,,. Hi-*. i.iijwM.'-I'.-I'ium'.BISvMU-'' ?><~- 

"■■■■■■ ^ ^Particles 

Wear angle 





Figure 31 -Cutting processes with worn conical bit 
(0 A = attack angle; 8 C = initial clearance angle; 6^ = initial tip 
angle.) 



reached. A hot smear of the molten material then forms 
on the surface of the sandstone. Further heat generation 
due to abrasion is considered to be minor since the contact 
area is lubricated by the molten material. With a steel tip, 
the smear is considered to be molten steel at 1,450° C. 
With a high-melting tip material such as tungsten carbide, 
the smear is usually considered to involve molten silica 
(Si0 2 ) at 1,710° C. Such temperatures are sufficiently high 
to ignite a methane-air mixture if the area and exposure 
time of the hot smear are adequate. 

Blickensderfer (9) investigated the frictional ignition of 
a methane-air mixture using a rectangular, 1-cm-wide tool 
to make a series of 0.005-cm-deep cuts at the same loca- 
tion in a 10-cm-long sandstone block. Ignition with a steel 
tool typically required about 75 cuts with a tool velocity of 
450 cm/s and 175 cuts with a tool velocity of 150 cm/s. A 
steel smear was left on the sandstone. A temperature of 
about 1,400° C was measured directly behind the tool for 
about 2 ms after passage of the tool, which then decreased 
to 1,200° C in an additional 2 ms. The steel smear was 
about 20 /im thick, and the top 0.5-/im layer of the smear 
was oxidized. 

Blickensderfer (35) developed a theoretical model of 
frictional ignition with a steel tool cutting into sand- 
stone. He assumed that (1) abrasion between the wear 
flat on the tool and the sandstone rapidly heated the 
surface of the wear flat on the steel tool to its melting 
point of 1,450° C, (2) a smear of molten steel was 
deposited on the sandstone and was maintained at its 



melting point until the tool passed on and exposed the 
molten smear, and (3) the exposed molten steel smear 
then cooled by heat conduction into the sandstone sub- 
strate. The steel smear theoretically was at 1,450° C for 
1.7 ms after exposure because of the heat released during 
solidification of the molten steel smear and 2.7 ms if the 
heat of oxidation of the 0.5-/xm oxide layer on the steel 
smear was included. With a tool velocity of 450 cm/s, the 
length of the molten smear was 12 mm and the area thus 
was 120 mm 2 . Blickensderfer concluded that such a hot 
surface (1,450° C for 2.7 ms with a 120-mm 2 area) 
reasonably agreed with the requirements noted earlier for 
ignition of a methane-air mixture with electrically heated 
surfaces. 

Blickensderfer considered that the likelihood of fric- 
tional ignition decreased as the area of the molten smear 
decreased. When the tool velocity decreased to 150 cm/s, 
the theoretical area of the exposed molten smear 
decreased to 40 mm 2 . He concluded that this decrease in 
molten area explained the increase in the number of cuts 
experimentally required to get ignition with the lower tool 
velocity. 

Cutler (37) similarly measured a streak temperature of 
about 1,400° C behind a tungsten-carbide-tipped mining bit 
cutting into sandstone. 

In the present study, the nature of the hot streak 
formed by a mining bit appears to be complex. Figure 4 
shows that the streak directly behind the bit initially is very 
hot: i.e., the film was overexposed for about 20 ms (5 cm) 
after passage of the bit because of the high local 
temperature. 7 The streak then cooled to show a series of 
discrete luminous hot spots about 1 cm or so apart on the 
surface of the sandstone, although luminous strips about 
5 cm long occasionally formed. The hot spots usually were 
about 100 mm 2 in area. Figure 32 gives another example 
of the series of hot spots formed by the bit, which in this 
case had passed downward out of the field of view. The 
hot spots at the bottom and top of figure 32 were about 
20 and 100 ms old, respectively, and were still hot enough 
to be luminous but did not give ignitions. 

The BCRNL study (18) indicated that water that im- 
pacted the sandstone surface further than 5 cm (20 ms) 
behind the bit was ineffective in preventing frictional igni- 
tion. Ignition therefore presumably involves the very hot 
spots in the hot streak within about 5 cm behind the bit, 
because the spots further than 5 cm behind the bit seem- 
ingly had cooled to a safe temperature. 

The hot spots formed here during an early nonigniting 
dry cut visually were similar to the hot spots formed during 
the next cut that gave ignition. Also, ignition always oc- 
curred during the bottom part of the igniting cut. Thus, a 
worn bit readily generates a nonigniting hot spot but then 
undergoes a small change and generates an igniting hot 
spot. 

Photometric measurement here of streak temperature was unsuc- 
cessful in that a streak temperature of about 1,100° ± 300° C was mea- 
sured behind the bit with both steel and tungsten-carbide tips. 



21 




Figure 32.-Spotty hot streak formed on surface of sandstone. 



The heat-generation process (or processes) taking place 
when a metal bit cuts into sandstone therefore must (1) be 
intermittent (to generate hot spots) and (2) be sensitive to 
small changes in the bit (to generate the igniting hot spot). 
A spotty type of hot streak and the qualitative similarity 
between nonigniting and igniting hot spots do not appear 
to have been reported by other investigators. 

Intermittent metal smears were left on the sandstone 
surface. X-ray analysis of the smears indicated that a tip 
made of Type 4140 steel left a steel smear while a cobalt- 
bonded tungsten-carbide tip left a cobalt smear. The 
metal smear was intermittent and did not appear to match 
the more regular sequence of luminous hot spots such as 
shown in figure 32. The formation of a metal smear 
implies melting of the metal tip. 

The luminous spotty hot streak formed on sandstone by 
a steel or carbide tip in a nitrogen environment visually 
was similar to the luminous streak formed in an air 
environment, and any exothermic oxidation of the molten 
metal smear appears to be of secondary importance. 8 



Frictional ignition due to sandstone-sandstone abrasion (35-55) is not 
addressed here. 



Heat generation during the cutting of sandstone with a 
metal bit presumably involves mechanical friction and 
depends upon the normal (perpendicular) force exerted by 
the wear flat of the bit on the sandstone substrate during 
the cutting process. With the present cutting technique, a 
tangential force is being applied to the bit by the rotating 
cutter drum in order to cut the sandstone, but no external 
normal force is being applied to push the bit against the 
sandstone target, as in rubbing-friction or drill-bit tests. A 
local normal force during the cutting operation does occur 
if a wear angle forms on the tip of the bit as shown in 
figure 31. 

An intermittent heat-generation process might be 
the formation of chips, where the partly formed chip 
(fig. 31) presses the wear flat against the sandstone sub- 
strate and thereby momentarily generates a high local 
normal force on the bit. Screen analysis of the particles 
formed during a 1-cm-deep cut indicated that 30 pet of 
the mass of the particles were large flakes with a nominal 
flake diameter greater than about 0.1 cm, indicating that 
chipping does occur. The normal force exerted by the 
bit during the present cutting operation has not yet been 
measured. 

While melting of the steel tip implies that the tempera- 
ture of the wear flat on the bit during cutting is about 
1,450° C, the temperature of the surface of the wear flat 
does not appear to have been directly measured. A tem- 
perature of about 400° C was measured with a thermo- 
couple 0.5 mm below the surface of the wear flat on a 
rectangular bit continuously cutting into sandstone (39). 
A similar low temperature of about 300° C was measured 
here with a thermocouple 1.2 mm below the wear flat on 
a conical bit intermittently cutting sandstone and also was 
measured with a thermocouple inside a drill bit (40). 
However, extrapolation of single values to the surface of 
the wear flat is of course impossible. 

A theoretical model of the temperature-time history of 
a conical steel bit having a wear flat and intermittently 
cutting into a sandstone block was developed by Edwards 
at the Bureau's Pittsburgh Research Center. The model 
involved (1) a constant rate of heating of the interface 
between the wear flat on the bit and the sandstone surface 
during the cutting part of the drum rotation due to fric- 
tional abrasion combined with cooling by heat conduction 
into the bit and the sandstone and (2) heat conduction 
into the bit and convective cooling of the bit during 
the remainder of the rotation cycle. A sawtoothed 
temperature-time curve resulted. The model used the bit 
geometry and drum rotation speed corresponding to 
the temperature-measurement test noted above and a 
constant-area wear flat of 0.7 cm 2 obtained in a separate 
ignition test that gave ignition in the seventh cut with this 
wear-flat area. The theoretical temperatures 1.2 mm 
inside the bit and at the surface of the wear flat on the bit 
depended linearly upon the heating rate. The heating rate 
at the surface of the wear flat was varied until the 
theoretical temperature 1.2 mm inside the bit matched 
the experimental temperature. When a heating rate 
of 90 cal/cm 2 • s was used, the theoretical internal 



22 





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Figure 33.-Theoretical temperatures inside conical bit and on 
wear-flat surface on conical bit 



temperature matched the experimental temperatures of 
200° C at the end of the 7th cut and 300° C after the 
20th cut (fig. 33). 

The corresponding theoretical temperatures of the 
surface of the wear flat were only 400° and 500° C after 
the 7th and 20th cuts, while the observed melting of the 
steel wear flat implies that the temperature of the surface 
of the wear flat was about 1,450° C. However, Ueda (41) 
recently reported that the temperature inside a grinding 
wheel abrading a steel target measured with a thermo- 
couple was considerably lower than the temperature 
measured with a pyrometric technique, and the validity of 
thermocouple measurements thus is uncertain. 

The formation of the wear flat would seem to be a 
critical feature of the ignition process. With a conical bit, 
when the bit attack angle decreased, ignition required 
fewer cuts with a new bit (fig. 8) and involved a larger 
area wear flat on the used bit. 

The wear rate of a steel-tipped conical bit was in- 
vestigated by measuring the area of the wear flat formed 
on a new bit in an air environment versus the number of 
cuts in the sandstone block. The area of the wear flat 
formed after a few cuts was measured by inking the wear 
flat, pressing the inked flat against graph paper, and 
counting the number of inked squares. A few additional 
cuts were then made, and the area of the wear flat was 



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NUMBER OF CUTS (N) 



100 



Figure 34.-Area of wear flat formed on steel-tipped conical bit 
versus number of cuts. (Solid and hollow symbols for the same 
e A indicate repeat tests.) 



remeasured, and so forth. The effect of bit attack angle 
8 A was investigated. A constant internal bit tip angle 6 T 
of 80° was used. 

Figure 34 plots the area of the wear flat a versus the 
number of cuts N for three attack angles (9 A = 45°, 53°, 
and 70°). The area increased approximately parabolically 
with N. The area also increased as 6 A decreased; e.g., 
after 20 cuts, the area was 0.5 cm 2 if 6 A = 70° and 1 cm 2 
if 6 A = 45°. Similar results were observed with brass and 
tungsten-carbide tips. 

The linear wear length £, perpendicular to the sand- 
stone surface, is related to the wear flat and the bit geo- 
metry 6 A and 6 T for a conical bit as shown in figure 35. 
The wear flat is oval in shape because of the slanted wear 
of the conical tip. Approximating the oval wear flat with 
a circle and assuming the wear flat is parallel to the sand- 
stone surface, £ is related to the wear-flat area a by sim- 
ple geometry, with £ = J a • { sin^yj /[tan (9 T /2) -J*]}. 
The parabolic a - N relations for various © A 's in figure 34 
convert to a single parabolic-type £ - N relation for the tip 
material, which is independent of 6 A . The large wear flat 
obtained with a small 8 A compared with the small wear 
flat obtained with a large 6 A thus appears to be a simple 
geometric effect and does not involve a variation of 
£ with 6 A . The value of £ does depend upon the nature 
of the tip material and the total length of cut L. Since the 
109-cm-diam drum is cutting into a 51-cm-high sandstone 
block, each cut is 54 cm long and L = 54 -N (in centi- 
meters). Figure 36 plots £ versus L for brass, steel and 
carbide tips. The wear rate of a tip is the tangent of an 
£ - L curve. Initial wear rates were high and depended 
upon the bit material. The wear rates then slowed and 
were similar for these various materials. 



23 




distance _il*y 



Sandstone 



Figure 35.-Geometry of linear wear distance with conical bit 
(e A = attack angle; 8j. = initial tip angle.) 



1.0 



i — r 




200 400 600 800 
TOTAL LENGTH OF CUT (L),cm 



,000 



Figure 36.-Unear wear distance versus cutting distance with 
conical bits. 



The rapid early wear of a drag-type drill bit was pre- 
viously reported (42-43). Neville and Crone (43) similarly 
observed that the area of the wear flat formed on a drill 
bit increased if the bit attack angle 6 A decreased and that 
the linear wear rate of the bit tip was independent of 6 A . 
They concluded that (1) wear was independent of the ex- 
ternal normal force (thrust) exerted on the drill bit 
because the bit is mainly crushing and not rubbing the 
rock and (2) any temperature effect due to heating of the 
drill bit was minor. 



The variation in the early wear rates of the present 
three materials and the similarity of the later wear rates 
suggest that different aspects of the wear process are 
important in the early and the later stages of bit wear, 
which remain to be explained. 

However, the area of a wear flat formed in a methane- 
air environment during an ignition test was about three 
times larger than the wear-flat area formed in an air 
environment with the same number of cuts; i.e., the wear 
length £ in a combustible methane-air environment was 
about two times the wear length in an air environment 
(fig. 36). The simplest explanation for this increase in 
wear rate in a methane-air environment perhaps is that a 
hot spot ignites the methane-air mixture to form a micro 
flame kernel that heats the surface of the bit (and also the 
sandstone) but is quenched by the bit and sandstone sur- 
faces and does not ignite the bulk methane-air environ- 
ment. The hot surface of the wear flat then wears faster 
than the warm surface of the wear flat because of heat 
generated only by friction; this process forms a large-area 
wear flat, which then forms a macro flame kernel that ig- 
nites the bulk environment. 

A working hypothesis for the mechanism of frictional 
ignition with a new frozen conical bit cutting sandstone 
thus might be as follows: 

1. The bit tip initially abrades very rapidly to form a 
small wear flat. The initial linear wear rate of the tip is 
slower with a harder (more durable) tip material. 

2. The tip of the wear flat forms chips of the sandstone 
target, and the wear flat abrades the remaining layer of the 
sandstone and the resulting sandstone particles. During 
chip formation, the normal force exerted by the bit on the 
sandstone target is momentarily very high, leading to the 
intermittent formation of a small hot spot on the surface 
of the sandstone. 

3. The small hot spot on the sandstone is about 
1,450° C and ignites a small flame kernel. The small flame 
kernel heats the wear flat and increases its wear rate 
but is quenched by the bit and the sandstone and does 
not propagate into and ignite the bulk combustible 
environment. 

4. The area of the wear flat on the bit tip increases 
because of frictional abrasion. The rate of increase of the 
wear-flat area depends upon the linear wear rate of the 
bit tip and the tip geometry. The linear wear rate is 
increased in a combustible environment because of heating 
of the tip by the flame kernel. Eventually, a large flame 
kernel ignited by a large hot spot formed by a large-area 
wear flat propagates into and ignites the bulk combustible 
environment. 

A similar mechanism should apply to frictional ignition 
with a rectangular-shaped bit except that the geometrical 
relation between the area of the wear flat and the linear 



24 



wear of the tip of the rectangular bit will be different from 
that shown in figure 35. 

Major features requiring clarification include hot-spot 
formation by chipping, quenching of the flame kernel, and 
cooling of the hot spots by the back spray. E.g., the bit 
geometry presumably should have a significant effect on 
kernel quenching and in "protecting" the hot spot from 
being cooled by the spray water drops. Detailed investi- 
gation of these features is planned. 

It should also be noted that an anti-ignition back spray 
may also achieve an antidust benefit. The dust cloud 
formed when a conical bit cuts into sandstone is shown in 
figure 37. With an antidust wet-head cutter drum such as 
discussed in reference 22, the drum-mounted water spray 
nozzles usually were located in the general vicinity of the 
bits and the water drops merely filled the cavity between 
the surface of the drum and the face. With an anti- 
ignition wet-head drum, the spray nozzles must be located 
behind the bit and must be carefully aimed to impact the 
mineral surface immediately behind the bit. The anti- 
ignition sprays thus are pointed directly into the dust cloud 
and should be more likely to capture the dust particles. 




Figure 37.-Dust cloud formed by bit cutting sandstone. 



CONCLUSIONS 



1. In the United States, frictional ignition usually 
involves a metal bit cutting into sandstone to form a hot 
streak on the surface of the sandstone. 

2. Frictional ignition always involves a worn bit having 
a wear flat on the tip of the bit; it seemingly can be ob- 
tained with almost any bit material but is much more likely 
to occur with a steel tip than with a tungsten-carbide tip. 

3. The likelihood of frictional ignition decreases with 
(1) a mushroom-shaped tungsten-carbide bit tip and (2) an 
increased bit clearance angle. A field test with conical- 
shaped bits in a coal mine supports these laboratory 
conclusions. Commercial versions of mushroom-shaped 
tungsten-carbide-tipped conical bits are available. 

4. The likelihood of ignition is not significantly 
decreased with a lower bit velocity in the range considered 
to be of practical interest. 

5. The likelihood of ignition with very worn bits 
significantly decreases if water spray nozzles are located in 
back of each bit and are carefully oriented so that the 
water spray impacts and cools the hot streak on the 
sandstone surface immediately behind the bit. A mini- 
mum water density at the impaction plane of about 
0.007 gpm/cm 2 is apparently required and can readily be 
achieved with commercial spray nozzles operated at 



moderate conditions (0.5 gpm, 50 to 100 psig). The like- 
lihood of ignition with all bits tested here was reduced by 
using a carefully designed back spray. Commercial 
versions of back spray and bit systems that use a high 
waterflow rate (1 gpm) are available. 

6. Replacement of worn bits must be scheduled. The 
frequency of bit replacement will depend upon the reme- 
dial technique but may range from every shuttle car load 
(if a mushroom bit and increased bit clearance angle are 
used) to every week (if a back spray is used). 

7. Formation of the hot streak appears to involve com- 
plex cutting-abrasion phenomena. Formation of local hot 
spots may be due to a momentary high normal force on 
the bit because of chip formation during the cutting pro- 
cess. The hot spot perhaps forms a micro flame kernel 
at the wear flat and sandstone interface, which heats the 
bit but does not propagate into the bulk environment. The 
heated bit would then wear more rapidly to form a large 
wear flat, which forms a macro flame kernel that ignites 
the bulk environment. 

8. An anti-ignition back spray may also achieve an anti- 
dust benefit because the back spray is aimed directly into 
the dust cloud formed by the cutter bit. 



25 



REFERENCES 



1. Wynn, A. H. A. The Ignition of Firedamp by Friction. Paper in 
Seventh International Conference of Directors of Safety in Mines 
Research. SMRE Rep. 42, 1952, 23 pp. 

2. Eisner, H. S. Discussion of paper by F. Powell and K. Billinge, 
" The Frictional Ignition Hazard Associated With Colliery Rocks." Min. 
Eng. (London), v. 134, July 1975, pp. 527-533. 

3. Elfstrom, R H. Explosion in No. 26 Colliery, Glace Bay, 
Nova Scotia, on February 24, 1979. Can. Dep. Labour, Apr. 1980, 
122 pp. 

4. Hartmann, I. Frictional Ignition of Gas by Mining Machines. 
BuMines IC 7727, 1955, 17 pp. 

5. Powell, F., and K. Billinge. Ignition of Firedamp by Friction 
During Rock Cutting. Min. Eng. (London), v. 134, May 1975, 
pp. 419-426. 

6. Powell, F. Ignition of Gases and Vapors. Ind. and Eng. Chem., 
v. 61, No. 12, 1969, pp. 29-37. 

7. Lobejko, A. The Ignition of Methane-Air Mixtures by Frictional 
Sparks. Health and Saf. Exec. Transl. Serv. Harpur Hill, Bunton, 
Great Britain, Transl. 8999, May 1980, 8 pp. 

8. Bartknecht, W. Preventive and Design Measures for Protection 
Against Dust Explosions. Paper in Industrial Dust Explosions, ed. 
by K. L. Cashdollar and M. Hertzberg. ASTM STP 958, 1987, 
pp. 158-190. 

9. Blickensderfer, R, D. K. Deardorff, and J. E. Kelley. Incendivity 
of Some Coal-Cutter Materials by Impact-Abrasion in Air-Methane. 
BuMines RI 7930, 1974, 20 pp. 

10. Larson, D. A, V. W. Dellorfano, C. F. Wingquist, and 
W. W. Roepke. Preliminary Evaluation of Bit Impact Ignitions of 
Methane Using a Drum-Type Cutting Head. BuMines RI 8755, 1983, 
23 pp. 

11. Liebman, I. and L. Cheng. Anti-Incendive Coal Cutting Bits. 
Pat. Appl. 319, 862, Nov. 1981. 

12. Cheng, L., I. Liebman, A. L. Furno, and R W. Watson. Novel 
Coal-Cutting Bits and Their Wear Resistances. BuMines RI 8791, 1983, 
15 pp. 

13. Cheng. L. Design of Anti-Incendive Conical Cutting Bits. Pres. 
at SME-AIME Annu. Meet., Las Vegas, NV, Feb. 27-Mar. 2, 1989. 
Trans. Soc. Min. Eng. AIME, in press. 

14. Cheng, L., A. L. Furno, and W. G. Courtney. Reduction in 
Frictional Ignition Due to Conical Coal-Cutting Bits. BuMines RI 9134, 
1987, 10 pp. 

15. NcNider, T., E. Grygiel, and J. Haynes. Reducing Frictional 
Ignitions and Improving Bit Life Through Novel Pick and Drum 
Designs. Paper in Proceedings of the Third Mine Ventilation 
Symposium. Soc. Mining Eng. AIME, Littleton, CO, 1987, pp. 119-125. 

16. Kocherga, N. G. Prevention of Methane Ignitions by the Use of 
Water Spraying During Operation of the Cutting Tools of Mining 
Machines. Health and Saf. Execu., Transl. Serv., Harpur Hill, Bunton, 
Great Britain. Transl. 7507, Nov. 1977, 7 pp. 

17. Powell, F, and K. Billinge. The Use of Water in the Prevention 
of Ignitions Caused by Machine Picks. Min. Eng. (London), v. 141, 
Aug. 1981, pp. 81-85. 

18. Agbede, R O, K. L. Whitehead, R L. Mundell, and R D. 
Saltsman. Frictional Ignition Suppression by the Use of Cutter-Drum 
Mounted Water Sprays. BuMines contract J0395040, Bituminous Coal 
Res., 1982; for inf., contact W. G. Courtney, BuMines, Pittsburgh, PA. 

19. Lewis, W. T, P. R Smith, and F. Powell. Circumstances 
Generating Incendive Sources in Rock Cutting. Paper Dl in 20th 
International Conference of Safety in Mines Research Institutes. 
SMRE, 1983, 11 pp. 

20. Cheng, L. Dynamic Spreading of Drops Impacting onto a Solid 
Surface. Ind. & Eng. Chem. Process Des. and Dev., v. 16, No. 2, 1977, 
pp. 192-197. 



21. Divers, E. F. Nonclogging Water Spray System for Continuous- 
Mining Machines. BuMines IC 8727, 1976, 10 pp. 

22. Strebig, K. C. Wet-Head Tests on Miners Concluded. Coal Min. 
& Process., v. 12, Apr. 1975, pp. 78-88. 

23. Courtney, W. G. Prevention of Frictional Ignition With Ripper- 
Type Continuous Mining Machines Using Water Sprays. Paper in 
Proceedings of the 3rd Mine Ventilation Symposium. Soc. Min. Eng. 
AIME, Littleton, CO, 1987, pp. 126-131. 

24. Merritt, P. C. The Wet-Head Miner Comes Back. Coal Age, 
v. 92, Nov. 1987, pp. 44-46. 

25. Taylor, C. D., and A. L. Furno. Evaluation of High-Pressure 
Front-Mounted Water Jets for Frictional Ignition Suppression. 
BuMines RI 9237, 1989, 8 pp. 

26. Roepke, W. W., B. D. Hanson, and C. E. Longfellow. Drag Bit 
Cutting Characteristics Using Sintered Diamond Inserts. BuMines 
RI 8802, 1983, 30 pp. 

27. Collin, W. D., and J. A. Kornecki. The Development and Use 
of Diamond Picks for Longwall Shearers at Secunda Collieries. Paper 
in Mining 85 Conference, Mining Productivity Through Reliability and 
Control. Inst. Min. Eng., Doncaster, England, v. 1, 1985, pp. 153-163. 

28. Titman, H. A Review of Experiments on the Ignition of 
Inflammable Gases by Frictional Sparking. Trans. Inst. Min. Eng. 
v. 115, 1955-56, pp. 535-557. 

29. Rae, D., B. Singh, and R Danson. The Size and Temperature of 
a Hot Square in a Cold Plane Surface Necessary for the Ignition of 
Methane. SMRE Res. Rep. 224, 1964, 35 pp. 

30. Cutler, D. P. The Ignition of Gases by Rapidly Heated Surfaces. 
Combust, and Flame, v. 22, 1974, pp. 105-109. 

31. . Further Studies of the Ignition of Gases by Transiently 

Heated Surfaces. Combust, and Flame, v. 33, 1978, pp. 85-91. 

32. Laurendeau, N. M. Thermal Ignition of Methane-Air Mixtures 
by Hot Surfaces: A Critical Examination. Combust, and Flame, v. 46, 
1982, pp. 29^9. 

33. Laurendeau, N. M., and R N. Caron. Influence of Hot Surface 
Size on Methane-Air Ignition Temperature. Combust, and Flame, v. 46, 
1982, pp. 213-218. 

34. Osburn, H. J. Wear of Rock-Cutting Tools. Powder Metall., 
v. 12, 1969, pp. 471-502. 

35. Blickensderfer, R Methane Ignition by Frictional Impact Heat- 
ing. Combust, and Flame, v. 25, 1975, pp. 143-152. 

36. Burgess, M. J., and R V. Wheeler. The Ignition of Firedamp by 
the Heat of Impact of Rocks. SMRE Rep. 46, 1928, 29 pp. 

37. Nagy, J., and E. M. Kawenski. Frictional Ignition of Gas During 
a Roof Fall. BuMines RI 5548, 1960, 11 pp. 

38. Rae, D. The Role of Quartz in the Ignition of Methane by the 
Friction of Rocks. SMRE Res. Rep. 223, 1964, 50 pp. 

39. Wingquist, C. F, and B. D. Hanson. Bit Wear-Flat Temperature 
as a Function of Depth of Cut and Speed. BuMines RI 9112, 1987, 
15 pp. 

40. Whitbread, J. E. Bit Temperatures in Rotary Drilling. Colliery 
Eng., (London), v. 37, Jan. 1960, pp. 25-59. 

41. Ueda, T., A. Hosokawa, and A. Yamamoto. Measurement of 
Grinding Temperature Using Infrared Radiation Pyrometer With 
Optical Fiber. J. Eng. Ind., v. 108, 1986, pp. 247-251. 

42. Fish, B. F, G. A. Guppy, and J. T. Ruben. Abrasive Wear 
Effects in Rotary Rock Drilling. Bull. Inst. Min. and Metall., No. 630, 
1959, pp. 357-383. 

43. Neville, H. F. C, and J. G. D. Crone. Wear of Rotary Drag Drill 
Bits in Granitic Rock. Trans. Inst. Min. and Metall., v. 71, 1961-62, 
pp. 249-263. 



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